The present invention relates to a process for the combined production of ammonia and urea in a plant comprising an ammonia synthesis reactor, a urea synthesis reactor and a urea recovery section.
In the following of the description and subsequent claims, with the term: xe2x80x9cprocess for the combined production of ammonia and ureaxe2x80x9d, it is intended to mean a single process that integrates the ammonia production process with the urea production process.
In other words, according to this technology, urea is producedxe2x80x94at least in partxe2x80x94by causing ammonia obtained in a synthesis reactor to react with carbon dioxide contained in a synthesis raw gas flow comprising among other things hydrogen and nitrogen coming, for instance, from a reforming section. The synthesis raw gas flow, free from carbon dioxide, is thereafter sent to the ammonia synthesis reactor.
Processes of this kind allow to eliminate, or in any case, to reduce to a remarkable extent, the decarbonation section of the synthesis raw gas flow, the separation section of the ammonia produced in the corresponding synthesis reactor, and the carbon dioxide compression section. Besides, energy consumption and investment costs resulting from a single integrated system may be substantially lower than those resulting from two separate processes for ammonia and for urea.
The need to provide an integrated process is particularly felt in all cases where all or in any case most of the ammonia is converted into urea by causing it to react with carbon dioxide obtained as a by-product in the preparation of the synthesis gas.
In the following of the description and subsequent claims, with the term: xe2x80x9curea recovery sectionxe2x80x9d, it is intended to mean the part of the plant downstream of the urea synthesis reactor, comprising generally one or two carbamate decomposers at medium pressure (about 18 bar a), respectively at medium and low pressure (about 4 bar a) and related carbamate condensers, whose function is to separate the produced urea from the reaction mixture coming from the corresponding synthesis reactor, allowing in this way to obtain a 60% to 75% concentrated urea solution.
The invention also relates to a plant for implementing the aforesaid process, as well as to a method for the simultaneous modernization of an ammonia production plant and a urea production plant.
In the following of the description and subsequent claims, with the term: xe2x80x9csimultaneous modernizationxe2x80x9d, it is intended to mean a modernization that concernsxe2x80x94at the same timexe2x80x94both an existing plant for ammonia synthesis and an existing plant for urea synthesis, for the purposes of their integration.
The integration between the production processes of ammonia and urea, wherein the carbon dioxide contained in the synthesis raw gas and the synthesis ammonia are caused to react, producing a carbamate aqueous solution to be sent to the urea synthesis reactor, involves on the one hand a simplification of the plantxe2x80x94with special reference to the ammonia decarbonation and separation sections and to the CO2 compression sectionxe2x80x94but on the other hand, a marked overloading of the sections correlated with the urea production, essentially due to the lack of formation heat and to the excessive molar ratio H2O/CO2 in the urea synthesis reactor, with an ensuing low conversion yield and high energy consumption.
As a consequence, in the field of combined production of ammonia and urea there is increasingly felt the need of providing processes allowing to increase urea conversion yield, in a simple way, with low operating and investment costs.
In order to meet the above requirement, several processes for combined production of ammonia and urea have been proposed in the field.
For instance, in U.S. Pat. Nos. 3,303,215 and 3,310,376, it is disclosed a process for the combined production according to the prior art wherein suitably purified liquid ammonia is fed to a urea synthesis reactor where ammonia is caused to react with carbon dioxide comprised in a synthesis raw gas including also hydrogen and nitrogen.
In the urea synthesis reactor, ammonia and carbon dioxide react forming ammonia carbamate which, in its turn, is transformed into urea by dehydration.
A first drawback of this process lies in that the high development of heat produced during carbamate production and the presence of inert gases (hydrogen and nitrogen) which reduce the partial pressure of ammonia and carbon dioxide makes it necessary to operatexe2x80x94in the urea synthesis reactorxe2x80x94at high pressures to keep reactants in a liquid phase, with ensuing high energy consumption and operating costs.
Moreover, because of the introduction in the urea synthesis reactor of a high amount of waterxe2x80x94for instance in the form of carbamate in aqueous solutionxe2x80x94to facilitate the absorption of carbon dioxide in the ammonia solution and the subsequent reaction into carbamate, the H2O/CO2 molar ratio in such synthesis reactor is relatively high and the conversion yield is unsatisfactory.
A further drawback lies in the structural and operating complexity of the urea synthesis reactor necessary for implementing the above described process, which must include a special unit for the separation of inert gases (hydrogen and nitrogen) from carbon dioxide and from ammonia in the vapour phase.
According to this process of the prior art, there is also provided a step of condensation and separation of the ammonia produced by unreacted gases, typical of ammonia production processes, which is rather demanding from the economic and energy consumption viewpoints.
In U.S. Pat. Nos. 3,349,126, 4,012,443, 4,013,718, and 4,320,103, it is disclosed another type of process according to the prior art, which comprises a separate section for carbon dioxide absorption and carbamate synthesis.
According to this process, ammonia coming from the corresponding synthesis reactor is separated from unreacted gasesxe2x80x94generally by absorption with water in a special absorption sectionxe2x80x94and sent to the carbamate synthesis section, wherein its reacts with the carbon dioxide contained in the synthesis raw gas flow coming from a reforming section, forming ammonia carbamate, which is sent to the urea synthesis reactor.
Also in this case, carbon dioxide absorption and the subsequent reaction into carbamate takes place in a milieu rich in water, which is then sent together with the carbamate to the urea synthesis reactor.
In addition, carbamate formation heat that is released during carbon dioxide absorption with the ammonia solution causes a strong evaporation of the latter, which involves the need of an additional recovery of ammonia at the outlet of the carbamate synthesis section, with ensuing problems of excessive dilution of the carbamate. At the same time, as the urea conversion reactor lacks carbamate formation heat, the operating conditions in said reactor become more difficult.
Otherwise said, according to this process of the prior art, producing carbamate outside the urea synthesis reactor involves not only the loss of the related formation heat, but also requires an addition of water, which is in contrast with the subsequent dehydration to urea, and therefore does not allow to obtain satisfactory conversion yields.
In conclusion, the processes for combined production of ammonia and urea according to prior art, besides requiring very complex plants for their implementation, and involving high investment and operating costs, as well as high energy consumption, do not allow in any case to obtain a high urea conversion yield due to the excessive H2O/CO2 molar ratio present in the corresponding synthesis reactor.
Because of these drawbacks, the above processes have not found till now a concrete application, in spite of the increasingly felt requirement in the field.
The problem underlying the present invention is to conceive a process for combined production of ammonia and urea, such as to allow on the one hand to obtain a high urea conversion yield, and to be, on the other hand, of simple implementation, with low operating and investment costs, and also with low energy consumption.
The above problem is solved, according to the invention, by a process of the aforesaid type, comprising the steps of:
submitting at least part of a flow comprising carbamate in aqueous solution coming from the urea recovery section to a partial decomposition treatment, to obtain a flow comprising ammonia and carbon dioxide in vapour phase and a flow comprising diluted carbamate in aqueous solution;
feeding said flow comprising ammonia and carbon dioxide in vapour phase to the urea synthesis reactor;
feeding said flow comprising diluted carbamate in aqueous solution resulting from said treatment step, a gas flow comprising hydrogen, nitrogen and carbon dioxide, preferably obtained by hydrocarbons steam reforming, and a flow comprising ammonia coming from the ammonia synthesis reactor to a carbamate synthesis section,
reacting said ammonia with said carbon dioxide in said carbamate synthesis section, to obtain a flow comprising carbamate in aqueous solution and a gas flow comprising hydrogen and nitrogen;
feeding said flow comprising carbamate in aqueous solution to said urea synthesis reactor;
feeding said gas flow comprising hydrogen and nitrogen to said ammonia synthesis reactor.
Advantageously, thanks to the process according to the present invention, and in particular to the step of partial decomposition of the carbamate coming from the urea recovery section, it is possible to send to the carbamate synthesis section a water-rich solution, and to send at the same time to the urea synthesis reactor a flow comprising ammonia and substantially anhydrous carbon dioxide, which allows to reduce the H2O/CO2 molar ratio in such reactor increasing therefore the urea conversion yield.
In this way, besides keeping a low H2O/CO2 molar ratio in the urea synthesis reactor, it is also possible to advantageously exploit at least part of the water comprised in the carbamate in aqueous solution coming from the urea recovery section, recycling it in a simple and economical manner to the carbamate synthesis section, in order to facilitate carbon dioxide absorption and to keep the carbamate produced in the form of an aqueous solution, preventing so an undesired crystallisation of the same.
A further advantage resulting from the present process lies in the fact that by sending to the urea synthesis reaction a gas flow comprising ammonia and carbon dioxide, it is possible to supply at least part of the reaction heat necessary for urea synthesis directly from the heat generated by the reaction between ammonia and carbon dioxide in the urea synthesis reactor (carbamate formation heat). By so doing, it is possible to eliminate the problem of the heat balance in the urea synthesis reactor, even in those cases when substantially all of the carbon dioxide comprised in the synthesis raw gas is transformed into carbamate in the specific synthesis section.
Therefore, the process according to the present invention allows to obtain in an extremely simple and effective manner a combined ammonia and urea production at low investment and operating costs, and also with low energy consumption and a high urea conversion yield.
Differently from the processes according to the prior art, the present process advantageously allows also to eliminate the burdensome step of ammonia separationxe2x80x94by condensation or by absorptionxe2x80x94from unreacted gases.
Actually, according to the present invention, ammonia and carbon dioxide are separated at the same time from the respective flows and caused to directly react in a single carbamate synthesis section by exploiting their high capacity of chemical reaction, obtaining a carbamate solution to be sent to the urea synthesis reactor.
Preferably, the flow coming from the ammonia synthesis reactor comprises ammonia in vapour phase, so that the ammonia carbamate synthesis can take place at least partly in the gas phase, with an extremely rapid reaction between ammonia and carbon dioxide, that does not require a prior absorption of carbon dioxide in the flow comprising ammonia.
If in the carbamate synthesis section a water amount should be required higher than the amount included in the diluted flow comprising carbamate in aqueous solution resulting from the treatment step, the process according to the present invention advantageously comprises the further step of feeding a flow comprising water coming from a urea concentration section to said carbamate synthesis section.
In this way, by recycling the water obtained in one of the sections downstream of the urea synthesis reactor, it is no longer necessary to send to the carbamate synthesis section a flow comprising water coming from outside the process, obtaining in this way a saving in operating costs.
In order to advantageously increase the urea conversion yield, the process according to the present invention further comprises the steps of:
submitting at least part of said flow comprising carbamate in aqueous solution obtained in said carbamate synthesis section to a partial decomposition treatment, to obtain a flow comprising ammonia and carbon dioxide in vapour phase and a flow comprising diluted carbamate in aqueous solution;
feeding said flow comprising ammonia and carbon dioxide in vapour phase to said urea synthesis reactor;
feeding said flow comprising diluted carbamate in aqueous solution resulting from said treatment step to said carbamate synthesis section.
In fact, by so doing it is possible to send to the urea synthesis section a substantially anhydrous flow comprising ammonia and carbon dioxide that allows to further reduce the H2O/CO2 molar ratio with the ensuing increase in the conversion yield, advantageously recycling to the carbamate synthesis section the water present in the carbamate flow coming from such section.
In order to control the temperature inside the urea synthesis reactor and to ensure optimum operating conditions for urea conversion, the process according to the present invention further comprises the steps of:
pre-heating a flow comprising recycled ammonia coming from a urea synthesis section; and
feeding said pre-heated flow comprising ammonia to said urea synthesis reactor.
In accordance with an alternative embodiment of the process according to the present invention, the temperature inside the urea synthesis reactor is controlled thanks to the fact of further comprising the steps of:
cooling a flow comprising ammonia and carbon dioxide in vapour phase resulting from said carbamate partial decomposition treatment;
feeding the so cooled flow to said urea synthesis reactor.
Both of the above alternatives allow to exercise a direct and effective control of the temperature in the urea synthesis reactor, allowing to supply exactly the amount of heat necessary for a high conversion yield.
In the first case, the urea synthesis reactor is fed with a suitably pre-heated flow comprising recycled ammonia, while in the second case a flow comprising ammonia and carbon dioxide in vapour phase is suitably cooled before being fed to the urea synthesis reactor.
For the implementation of the aforesaid process, the invention advantageously provides a plant for combined production of ammonia and urea, comprising:
an ammonia synthesis reactor, a carbamate synthesis section, a urea synthesis reactor, a urea recovery section and a carbamate decomposition section;
means for feeding at least a part of a flow comprising carbamate in aqueous solution coming from said urea recovery section to said decomposition section;
means for feeding a flow comprising ammonia and carbon dioxide in vapour phase obtained in said decomposition section to said urea synthesis reactor;
respective means for feeding a flow comprising diluted carbamate in aqueous solution obtained in said decomposition section, a gas flow comprising hydrogen, nitrogen and carbon dioxide, coming preferably from a hydrocarbons steam reforming section, and a flow comprising ammonia coming from said ammonia synthesis reactor to said carbamate synthesis section;
means for feeding a flow comprising carbamate in aqueous solution obtained in said carbamate synthesis section to said urea synthesis reactor;
means for feeding a gas flow comprising hydrogen and nitrogen obtained in said carbamate synthesis section to said ammonia synthesis reactor.
According to a further aspect of the invention, there is also provided a method for the simultaneous modernization of an ammonia synthesis plant and a urea synthesis plant, comprising respectively an ammonia synthesis reactor and a urea synthesis reactor and a urea recovery section, characterized in that it comprises the steps of:
providing a carbamate synthesis section and a carbamate decomposition section;
providing means for feeding at least part of a flow comprising carbamate in aqueous solution coming from said urea recovery section to said decomposition section;
providing means for feeding a flow comprising ammonia and carbon dioxide in vapour phase obtained in said decomposition section to said urea synthesis reactor;
providing respective means for feeding a flow comprising diluted carbamate in aqueous solution obtained in said decomposition section, a gas flow comprising hydrogen, nitrogen and carbon dioxide, preferably coming from a hydrocarbons steam reforming section, and a flow comprising ammonia coming from said ammonia synthesis reactor to said carbamate synthesis section;
providing means for feeding a flow comprising carbamate in aqueous solution obtained in said carbamate synthesis section to said urea synthesis reactor;
providing means for feeding a gas flow comprising hydrogen and nitrogen obtained in said carbamate synthesis section to said ammonia synthesis reactor.
Thanks to the aforementioned method of modernization that combines an existing ammonia plant and an existing urea plant, it is possible to obtainxe2x80x94in a simple and economical wayxe2x80x94a high urea conversion yield and at the same time drastic reductions in operating costs and energy consumption.
The characteristics and advantages of the inventions are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the attached drawing.